1. Technical Field
The present invention relates to tunable optical sources.
2. Description of Related Art
Semiconductor laser diodes are known as optical sources. Various techniques are used to obtain single mode, narrow linewidth operation which is desirable in applications such as communications. For instance, unmodified edge emitting laser diodes typically operate with several longitudinal modes lasing simultaneously, leading to low coherence and large linewidths. A technique known for use with edge emitting laser diodes is to use a grating to act as a wavelength filter in providing feedback to the lasing cavity to obtain single mode operation. Examples are the distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers in which a grating extends at least partially along the length of the lasing cavity, or a neighbouring cavity in a monolithic arrangement, being formed for instance in the surface of a substrate supporting the lasing cavity and any neighbouring cavity.
An external cavity laser (ECL) is another known arrangement which uses wavelength selective feedback to obtain single mode operation. In this case, the lasing cavity is coupled via an end facet to a further cavity that contains a wavelength-selective feedback mechanism to provide feedback to the lasing cavity. The wavelength-selective feedback mechanism may for instance comprise individually mounted wavelength-selective optics in free space.
It is also known to use a DBR in an optical fibre as an external cavity providing tunable feedback to a laser diode. This is disclosed for example in U.S. Pat. No. 4,786,132. It is further known to use a distributed grating in a planar silica waveguide as an external cavity providing tunable feedback. This is disclosed for example in U.S. Pat. No. 5,870,417.